The receiver of the present invention is intended to respond to United States Navy Satellite Navigational Positioning System (NNSS) transmissions for precisely locating fixed or movable observation points on the surface of the earth. The NNSS system uses multiple orbiting Transit satellites which transmit orbit definition data at repeated time intervals. The basic NNSS system is described in U.S. Pat. Nos. 3,191,176 and 3,172,208.
As described in U.S. Pat. No. 3,172,208, typically, each Transit satellite's orbit is determined optically, or by other means of observation, and once the satellite's orbit is known with precision, the parameters describing the orbit are transmitted to the satellite for rebroadcast by the satellite. The observer at an unknown location receives the Doppler signals and orbital parameters from the satellite, and these data are supplied to a computer. The computer computes the satellite track from the orbital parameters, assumes various values for the earth's co-ordinates of the unknown location, and fits a theoretical Doppler signal curve to the actual Doppler signal curve. When the theoretical curve is fitted to the actual curve, the earth's co-ordinates of the observer's location will have been determined with a high degree of accuracy.
The Transit satellites in the NNSS system are in circular, polar orbits, about 1,075 kilometers high, circling the earth every 107 minutes. This constellation of orbits forms a "bird cage" within which the earth rotates, carrying the observer past each orbit in turn. Whenever a satellite rises above the horizon, the observer has the opportunity of obtaining a position fix. The average time interval between fixes with the existing five satellites which presently make up the NNSS system varies from about 35 to 100 minutes, depending upon latitude.
Each Transit satellite in the NNSS system includes a stable oscillator from which all signal frequencies are derived. A 150 MHz VHF carrier and a 400 MHz UHF carrier are derived from the oscillator. The carriers are phase modulated by digital signals representative of the orbit definition data, these data being transmitted during a repeated precisely timed two-minute time interval in the form of multi-bit words. The data are clocked at a selected clock frequency which is also derived from the stable oscillator. The clock generated by the satellite oscillator is adjustable from a ground station, to provide a precisely synthesized clock, which accurately defines each of the repeated two-minute time intervals during which the orbit definition digital data signals are transmitted.
The NNSS system includes tracking stations, and each time a Transit satellite passes within the line of sight of a tracking station, the tracking station receives the phase modulated 150 MHz VHF signal and the phase modulated 400 MHz UHF signal transmitted by the satellite. The tracking station measures the Doppler frequency shift caused by the satellite motion and records the Doppler frequency as a function of time, this frequency being the difference between the frequency of a stable local oscillator at the tracking station and the frequency of either one of the satellite carrier signals. The Doppler information is sent by the tracking station to a computing center where the data are used to determine each satellite's orbit, and to project each orbit many hours into the future. The computing center forms a navigational message from the predicted orbit, and this navigational message is sent to a plurality of injection ground stations. At the next opportunity, one of the injection ground stations transmits the navigation message to the appropriate satellite. Each satellite receives a new message about every 12 hours, although its memory capacity is 16 hours.
Unlike ground-based radio location systems which determine position by simultaneous measurements of signals from several fixed transmitters, NNSS measurements are made with respect to sequential positions of each Transit satellite as it passes the observer. This process typically requires from 6 to 16 minutes during which time the satellite travels from 2628 to 7008 kilometers providing an excellent base line. Because NNSS measurements are not instantaneous, motion of the observer's position during the satellite pass must also be considered in the calculations. Also, because the Transit satellites are in constant motion relative to the earth, simple charts with accurate lines of position are very difficult to generate. Instead, each satellite transmits a message which permits its position to be calculated to fractional meter accuracy as a function of time. By combining the calculated satellite position range difference measurements between these positions, that is Doppler counts, and information regarding the motion of the observer's position, an accurate fix of the observer's position can be obtained. Because the calculations are both complex and extensive, a small digital computer is required.
The NNSS system is one of the most dependable and reliable navigational positioning systems in existence at the present time, and it is considered the most accurate system for worldwide geodetic surveying. The available survey accuracy from the NNSS system is being constantly improved through more precise orbit predictions and through improved apparatus capabilities. Improvements have also been made in the software, and in the statistical message used to process the data by the observer's recovery system. However, no substantial advances have been made in recent years on the capabilities of the observer's receiver to recover the satellite transmissions with a high degree of accuracy.
A major source of error in the overall NNSS system is the inability of the receiver to extract accurately the satellite timing information. One reason for this is that present day prior art NNSS receivers extract the two-minute satellite timing information by recovering the synthesized clock from the demodulation products of the received satellite signals. However, such a technique produces timing errors due to frequency drifts of the local oscillator of the receiver, and also because of the high signal-to-noise ratio in the demodulation products from the demodulated signals which are at an extremely low amplitude level.
More specifically, in the prior art NNSS receivers, one of the satellite radio frequency carriers is demodulated and the synthesized satellite clock is recovered to enable the receiver to extract precise time information identifying the two-minute timing interval of the received data. However, in order to recover the satellite's synthesized clock, the prior art receiver depends on a local clock oscillator to effectuate the demodulation process, and with a phase-locked loop circuit recover the synthesized clock from the demodulation products. Prior art receivers are inaccurate since they depend upon the local clock oscillator and do not recover the true time slope and corrections programmed into the satellite clock by the injection station.
The NNSS satellite receiver of the present invention, on the other hand, derives the actual satellite clock directly from one of the radio frequency carriers, rather than deriving the synthesized clock from the demodulation products of the carrier, and its recovery process is not dependent upon a local clock. Through the technique of the present invention, the effects of long term drift of the local receiver oscillator, and detector signal-to-noise ratio errors, and other sources of error in the extraction of the two-minute time interval information from the received satellite signals are eliminated. Any drift in the oscillator of the satellite is recorded by the United States Naval Observatory, and is contained in the transmitted satellite signals. This enables the receiver to make appropriate corrections for drifts in the satellite clocks.
The principal objective of the present invention, therefore, is to provide a receiver for use in the NNSS system which achieves more accurate time interval information recovery from the received satellite signals, as compared with the prior art receivers, so as to achieve highly accurate geodetic positioning and surveying capabilities in the system.
As explained above, the operation of the receiver of the invention is predicated on the principle which invokes the extraction of both time and Doppler frequency signals from the received radio carrier, rather than deriving the time signals by the use of a local clock and detected modulation of the radio frequency carrier. The receiver of the invention eliminates to all intents and purposes the effects of time interval measuring errors for positioning accuracies of the system in the centimeter range. The receiver of the invention is capable of recovering precise timing information from the transit satellites to the extent that significant errors due to the satellite correction operations and off-sets are corrected in the receiver.